The present invention relates to a surface acoustic wave device using lithium tetraborate (Li.sub.2 B.sub.4 O.sub.7) which is a piezoelectric material.
In general, an interdigital electrode is formed on a substrate (elastic base) of a piezoelectric material in a surface acoustic wave device. A surface acoustic wave is propagated at a surface of the elastic base. The surface acoustic wave device of this type has a filtering function. The filtering characteristics can be arbitrarily changed by changing the electrode pattern. For this reason, the surface acoustic wave device has been applied to a surface acoustic wave filter, a surface acoustic wave resonator, and a surface acoustic oscillator.
While the electromechanical coupling coefficient K.sup.2 is required to be large in such a device, the center frequency of the filter or the resonant frequency of the resonator is also required to remain constant irrespective of a change in temperature. A temperature coefficient of delay time (to be briefly referred to as a TCD hereinafter) is used as a criterion of stability of the center frequency or the like against a change in temperature. A small TCD is required to stabilize the center frequency. Delay time is a time interval taken for the surface acoustic wave to propagate between two adjacent electrode elements of the interdigital electrode formed on the substrate. The TCD is a rate of change in delay time with respect to temperature. The electromechanical coupling coefficient K.sup.2 is one of the constants which determines an energy conversion efficiency so as to convert an electric signal to a surface acoustic wave.
FIG. 1 is a graph showing the characteristics of substrates made of various piezoelectric materials wherein the TCD is plotted along the axis of abscissa and the coupling coefficient K.sup.2 is plotted along the axis of ordinate. 128.degree.Y--X.LiNbO.sub.3 (lithium niobate) has a large coupling coefficient K.sup.2. But this material is not preferred as a surface acoustic wave element since its absolute value of TCD is also large. In the case of X--112.degree.Y.LiTaO.sub.3 (lithium tantalate), its coupling coefficient K.sup.2 is as large as about 0.8%, but its absolute value of the TCD is as large as about 20 ppm/.degree.C. This material is not preferred as a surface acoustic wave element. The TCD of ST-cut crystal is zero and the K.sup.2 thereof is as small as 0.2% or less. This material is difficult to use as a surface acoustic wave element. Unlike these materials, 25.degree.X--Z.Li.sub.2 B.sub.4 O.sub.7 (lithium tetraborate) has a coupling coefficient K.sup.2 as large as about 1% and a TCD of zero. For this reason, lithium tetraborate has received much attention as a piezoelectric material for a surface acoustic wave element. "25.degree.X--Z" indicates that the cut surface of the substrate is perpendicular to a rotated X-axis obtained by rotating the X-axis through 25.degree. about the Z-axis toward the Y-axis, and that the propagation or transfer direction of the surface acoustic wave is parallel to the Z-axis. "25.degree.X--Z" may be transformed to Eulerian angles (115.degree., 90.degree., 90.degree.).
However, since surface acoustic wave energy is concentrated on the surface of the substrate, the propagation characteristics of the surface acoustic wave change, depending on the surface state of the substrate. In the surface acoustic wave device, a metal such as aluminum (Al) must be deposited on the surface of the substrate to form an electrode. For this reason, even if a lithium tetraborate substrate (TCD: 0) is used, the TCD changes from zero in the surface acoustic wave device having the aluminum electrode film formed on its substrate. In particular, when a lithium tetraborate single crystal substrate is used, the surface state of the substrate greatly influences the propagation characteristics of the surface acoustic wave. Therefore, no conventional surface acoustic wave device having a substantially zero TCD has become commercially available.